1. Field of the Invention
The invention relates generally to the field of optics, and more particularly to substrates and wafers of optical devices.
2. Description of Related Art
Lithium Niobate (LiNbO3) is a type of ferroelectric materials in which the second-order nonlinear optical properties, including electro-optic and nonlinear optic properties, are intrinsically related to the crystal orientation, some time referred to as poling orientation or domain. One can purchase LiNbO3 substrates or wafers commercially that are up to 5″ in diameter which are already poled during or after the crystal growth. These wafers thus present single domain structures, i.e. the same c- or z-axis orientation over the whole volume.
In a conventional solution, the Ti (Titanium) indiffusion is accompanied by Li2O outdiffusion from the crystal surface. The Li2O outdiffusion is usually accompanied by domain inversion at the z+ crystal surface. When the domain inversion on the z+ crystal surface occurs, the result is usually a thin (up to few tens of μm) domain inverted layer, which is oppositely oriented with respect to the initial crystal domain structure. The closer the temperature to the Curie point (1145° C.), the more significant the formation of the thin domain layer on the z+ surface.
One shortcoming in this approach occurs after Ti indiffusion when it is necessary to carry out a grinding step on the z+ surface of the crystal in order to remove a thin domain inverted layer, which brings the crystal to a thinner single domain structure with optical waveguides on the z− surface. After removal of this thin layer, the crystal can be poled.
Besides being expensive and time consuming, the grinding can be detrimental for the mechanical resistance of the crystal. In particular, most of the processing is performed at a wafer level before dicing into chips. Any crystal weakness introduced by grinding, it may result in breakage of the wafer during subsequent processing steps and loss of all the chips.
Accordingly, there is a need for a method and structure for electric field poling of Ti indifussed LiNbO3 substrates without the use of grinding process.